Many optical systems use one or more refractive optical elements. These optical elements fall into a few canonical forms, e.g. lenses and prisms, and comprise two or more index of refraction discontinuities. Refractive optical elements can in turn be combined to form refractive optical systems such as collimators, focusing optical system, microscopes, telescopes, projection lenses, and interferometers. These elements are chosen to achieve system functions and to meet performance requirements. Precision refractive optical systems can be used in collimated laser applications, such as metrology, and imaging applications, such as photolithography. A refractive telescope, for example, typically comprises more than one lens element and is designed to achieve light collection and magnification in imaging applications, or beam expansion or reduction in collimated laser applications. Because of the precise optical operation, a high performance system, such as a collimator, focusing optical system, expander, or reducer, also requires calibration and testing during the production process to ensure that it will work for its intended purpose. Production and calibration is typically and most desirably performed in an ambient environment that is of standard atmospheric conditions. Operation of the system, however, may occur under different ambient conditions, such under vacuum or near-vacuum conditions (space for example) or in a fluid environment such as underwater. As one of ordinary skill in the art appreciates the ambient index of refraction changes with changes in pressure, temperature, gaseous composition, etc. Accordingly, an unfortunate situation arises where a collimator performs differently under the ambient conditions of production, test and calibration, which are at standard atmosphere, when compared to the ambient conditions of intended operation, which may not be at standard atmosphere.
An existing solution to the different ambient conditions between production and operation is to present a corrective optic during production, test and calibration that corrects for the differences expected between production and operation ambient conditions. The corrective optic is used during production and is then removed prior to intended operation. While this solution is far preferable to no corrective measures, it is indirect and introduces more uncertainties.
In addition, some systems are so sensitive that even operational ambient changes create unacceptable perturbations.
There remains a need, therefore, for a collimator that is insensitive to changes in ambient environment conditions.